Available-For-Sale Investments in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	$ 172,301	$ 39,721
Less Than 12 Months, Unrealized Losses	(149)	(15)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	172,301	39,721
Total Unrealized Losses	(149)	(15)
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	14,608	9,802
Less Than 12 Months, Unrealized Losses	(24)	(7)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	14,608	9,802
Total Unrealized Losses	(24)	(7)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	7,484
Less Than 12 Months, Unrealized Losses	(5)
12 Months or Greater, Estimated Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total Estimated Fair Value	7,484
Total Unrealized Losses	(5)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	145,202	29,919
Less Than 12 Months, Unrealized Losses	(119)	(8)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	145,202	29,919
Total Unrealized Losses	(119)	(8)
Securities of government-sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	5,007
Less Than 12 Months, Unrealized Losses	(1)
12 Months or Greater, Estimated Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total Estimated Fair Value	5,007
Total Unrealized Losses	$ (1)